I also recall another member here (+Doctor Octoroc) working on a 3D model of his own which incorporates a spring mechanism like those gag knives you see in some joke shops.
If you're looking for a really cheap solution that's not as accurate, you could buy a toy knife for a couple of pounds/dollar/insert currency here, like this one:
Personally, I'm planning on creating a wood replica to display with my other props.
Lastly, I'm not sure exactly (or how other Joker cosplayers get away with it) but many countries are really strict about anything that even resembles a real weapon, so you'd have to be careful about carrying a realistic looking knife around.